The invention relates to a process and an apparatus for determining the completion of a starting hole machined through a workpiece, where the starting hole is produced by spark erosion on an electroerosion machine using a starter electrode.
Prior to the actual spark erosive working of a workpiece, it is known to machine a starting hole through the workpiece by spark erosion. For this purpose an electroerosion machine is used having a starter electrode with adequate inherent rigidity. The starter electrode and workpiece are connected to the output of an electric supply device, typically a pulse generator. The circuit is closed through the working zone, i.e. the gap between the tip of the starter electrode and the workpiece. For producing the starting hole, the starter electrode is generally moved by a motor-driven spindle in a vertical direction through the workpiece or drill hole. The working gap or drill hole is frequently rinsed or cleaned in that pressurized fluid is pumped from the outside through the hollow starter electrode. As the spark erosive machining of starting holes is well known, reference can be made to the relevant literature to avoid unnecessary explanation here.
In the presently known systems for the production of starting holes on electroerosion machines, there exists the problem of determining the actual completion of the hole. When producing a single starting hole, the continuous monitoring of the drill hole is usually left to the operator. As soon as the operator determines the actual completion of the bore, i.e. the completion of the starting hole, the equipment is stopped. In the digitally controlled production of starting holes, the feed or advance that was imparted to the starter electrode was used as an evaluation quantity for determining the completion of the starting hole. This was essentially based on the idea that the starting hole is completed if a total feed corresponding substantially to the workpiece thickness is imparted to the starter electrode, optionally taking into account the wear of the starter electrode during the drilling process. However, the problem arose that electrode wear is not constant and can instead vary between 50 and 200 percent. The wear is also dependent on the temperature, the pollution or contamination state in the drill hole, the pressure of the rinsing fluid, the materials used for the electrode and the workpiece, as well as other factors. The generally unforeseeable extent of the wear is particularly serious if several starting holes are to be produced with the same starter electrode. Thus, due to the unknown extent of the wear the constant change in the starter electrode length is also unknown. In the case of several successively made starter holes, this can lead to obtaining only a blind hole instead of a completely machined starting hole. A blind hole could fundamentally be avoided in that from the outset a fictitious rate of electrode wear was programmed into the digital control of the drilling movement, which wear rate was well above the average wear. However, this measure suffers from the disadvantage that the electrode may still be driven by its feed mechanism after it has already passed through the workpiece and has already abutted against another workpiece or machine part, e.g. a clamping member. The latter situation can then lead to the destruction of the starter electrode.
For the aforementioned reasons, a sensible preprogramming of the starter electrode movement is difficult, if not impossible, to perform under economic considerations.